This invention relates to a lost foam casting process for making a metal casting having a bore defined by pore-free metal. More particularly, this invention relates to a vacuum chill device receivable within a bore of a vaporizable pattern employed in lost foam casting, which device withdraws pattern decomposition vapors from the bore during casting to accelerate solidification of metal about the bore and thereby reduce porosity in the bore wall.
A foundry mold for a lost foam metal casting process comprises an expendable pattern embedded in a body of unbonded sand particles. Molten metal is cast into the mold to decompose and replace the pattern, whereafter the metal solidifies to form a product casting that duplicates the pattern. The pattern is formed of a polymeric foam material, such as expanded polystyrene, that vaporizes at metal casting temperatures. Thus, the casting process is accompanied by generation of voluminous pattern decomposition vapors that vent into the surrounding sand body. Pattern decomposition vapors that do not vent may become entrapped in the metal and form pores. Also, the metal may exhibit shrink microporosity resulting from thermal contraction during solidification.
Ryntz et al U.S. Pat. No. 4,520,858, issued to in 1985, shows a lost foam casting process wherein a chill member is adhesively bonded to the pattern prior to casting. The chill preferentially accelerates solidification of metal in the adjacent casting region to reduce shrink porosity therein.
A principal advantage of the lost foam process is the duplication of bores in the product casting without special coring. A bore formed in the pattern fills with sand when the pattern is embedded in the mold, whereafter pattern replacement metal is shaped against the sand to define a bore in the product. Porosity is also a problem in bore walls of lost foam castings. Ruhlandt et al U.S. Pat. No. 4,706,732, issued to in 1987, shows a chill device for insertion into a bore. The chill device comprises fins that contact the bore wall and extract heat to reduce shrink porosity. The fins are spaced apart to provide channels for venting pattern decomposition vapors to avoid entrapment that might otherwise result in porosity.
It is known to apply a vacuum to the mold during lost foam casting to facilitate removal of pattern decomposition vapors. The vacuum has heretofore been applied to the mold generally, for example, through a plenum communicating with the mold through a screen bottom wall and is alternately used to inject air upwardly through the sand bed to fluidize the sand particles for embedding a pattern or removing a casting. It has now been found that preferential removal of pattern decomposition vapors from sand regions within a bore is effective to accelerate solidification of the metal cast thereabout in a manner similar to a chill member, and thereby reduces shrink porosity in the bore wall.
Therefore, it is an object of this invention to provide an improved lost foam casting mold comprising a vaporizable pattern embedded in an unbonded particulate body and having a bore for shaping a corresponding bore in a product casting, which mold further comprises means for preferentially removing pattern decomposition vapors from within the bore during casting to not only avoid entrapment, but also to accelerate solidification of cast metal about the bore relative to remote sections of the casting, to thereby reduce porosity in the bore wall, including in particular shrink microporosity.